This relates to an improved method and apparatus for echo control in data transmission circuits which will provide maximum data throughput with minimal increase in hardware.
The primary source of echo on a telephone network is a circuit mismatch at a device called a Hybrid which is the interface between "two wire" and "four wire" circuits. The two wire circuits carry transmission in both directions and connect individual telephones to their central offices and also interconnect central offices when the distance between them is very short. Four wire circuits provide separate paths for each direction of transmission. Long distance trunks are always four wire circuits because signal processing (multiplexing, amplification, etc.) can only be done with separate paths for each direction.
Ideally, the Hybrid should pass all the signal on the incoming four wire channel to the two wire circuit with no leakage into the outgoing four wire channel. Unfortunately, this is not the case and the signal leaking across the Hybrid returns as an echo to the person talking at the other end of the four wire circuit. This type of echo is called "talker echo" since the talker hears his own voice delayed by the round trip delay between his telephone and the Hybrid at the far end.
Most full duplex data sets of modems are not susceptible to talker echo because of frequency separation of the transmit and receive signals. For half duplex data sets, however, the return echo from the outgoing transmission may be received and passed on to the terminal as valid data. Coping with this problem has become more difficult with the introduction of satellite circuits that have significant round trip propagation delays (RTPD) in excess of 600 milliseconds (msec.) compared to delays of less than 50 msec. on land microwave circuits and 100 msec. on cable connections. Such extremely long delays significantly decrease two-way data throughput since there must be a delay equal in length to the RTPD between the time one terminal ends transmission and the time it begins to receive a response. These delays are especially troublesome in higher speed data sets where the length of a typical data message is relatively short (e.g., 10 msec.).
Historically, half duplex data sets have dealt with echoes by squelching the receiver until all echoes have passed. The receiver ignores energy on the line while the transmitter is sending and remains squelched until it sees a gap of energy, making it blind to talker echoes. However, as emphasized in U.S. Pat. No. 4,394,767 which is incorporated herein by reference, this "blind squelch" method has the disadvantage of requiring the communication system to assume that the RTPD is 600 msec. or more since the system does not know if the data sets are interconnected by satellite or by shorter land circuits. As a result, the transmitter will send a filler to ensure that the message is at least 600 msec.; and the minimum time between successive data transmissions on the system (i.e., minimum turnaround time) must be on the order of 600 msec.
A further complication has arisen, however, with the introduction of echo cancellers into satellite communication systems. Echo cancellers, as the name implies, control the echo by synthesizing a replica of the echo and subtracting it from the actual echo. Because the echo canceller has no prior knowledge of the characteristics of the circuit, the echo canceller must be adaptive; and to adapt the echo canceller to the circuit, a training preamble is required. The use of such a preamble, however, causes a delay before the circuit is properly conditioned for communication. While this preamble can be realitvely short if the equipment used is that of some manufacturers such as General DataComm Industries, Inc., the assignee of the present invention, conventional AT&T equipment requires a preamble of up to 500 msec. Because it is difficult to determine in advance if a particular call will use a satellite or terrestrial link and what echo control equipment might be in the link, in practice it is necessary to provide a lengthy preamble to train any echo canceller that may be present and to squelch the receiver until after the time that a data echo would return. As a result, the minimum turnaround time is on the order of 1.2 seconds. For this reason, the blind squelch method, although technically possible, is not a good solution.
Another data set echo control technique that is widely used is the echo suppressor that is commonly found on trunks of more than 3000 kilometers. These devices are voice operated switches which allow energy to pass only in one direction, eliminating a reflected echo. Modems usually disable suppressors and use the echo control methods mentioned above. While the suppressors could be used to eliminate echo, such use would introduce a different problem, because the suppressors require approximately 100 ms to reverse their transmission direction. This turnaround time chops off about 100 millisecoonds from the front end of the data transmission, a problem that is especially acute in modems with adaptive equalizers since the first part of the transmission is required to "train" their equalizer.
The modem can avoid this problem by sending energy such as unmodulated carrier to reverse the echo suppressors before beginning data transmission. With such a technique, which is called "sacrificial carrier," a delay of about 150 msec. is required before data transmission can begin. Obviously, this smaller delay is preferred to the 600 msec. transmission required by the blind receiver squelch method. However, because no attempt is made to determine the need for a sacrificial carrier, turn around time will always be degraded, even when an echo suppressor is not present. Moreover, end-to-end compatibility may be a problem because the modems on both ends have to use this echo control technique for it to work. The modem therefore has to determine if the far end modem is also compatible with this method. This problem, which is technically possible to solve, does require a cumbersome preliminary handshaking procedure which may also confuse the terminal equipment.